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Room Temperature & Humidity Control

Dear All,

I am controlling a store room. The Room, I need to Maintain a the temperature at 23 Degree C and maintain humidity at 55%.
My control is: one chiller is use to maintain temperature and dehumidifier. When the room is hot motorized valve will open. When Humidity is high motorized valve also open. and one heater will heat the room to maintain temperature, in case dehumidifying.
With this control I found that I can control temperature and Humidity only when the outside air is cool(13 to 23 Deg C). But when the outside air temperature is high(25 to 38 Deg C) the humidity cannot be controlled.

This room is located in a quite different temperature change (13 to 38deg C).

Thank you for your Idea this control.
Anyone have experience to control this system?

What temperature is your chilled water? If it is too cold, you won't get a long enough cycle on your coil and will not be able to dehumidify in warmer weather.
Does the chiller supply other loads as well or is it just for that one room?

More on the type of system would be nice. Is this an air handler that supplies a single room? Single/Dual Duct with return? Is it a fan coil with a seperate heating unit? If you have a duct temperature you can control it may be of some help to use a cascade loop.

Also the capacity of the chilled water coil just may not be enough under high OA temp/humidity for the airspace being controlled. Is the valve operating at 100% for a long period while high outside air temp? Is a lot of reheating happening under this condition?